Abstract
Abstract. An integral response model is proposed to describe the relationship between geomagnetic activity (aa index) and solar activity (represented by sunspot number Rz): The aa at a given time t is the integral of Rz at past times (t'≤t) multiplied by an exponential decay factor of the time differences (e−(t−t')/τ), where τ is the decay time scale (~40 months). The correlation coefficient of aa with the reconstructed series based on this model (rf=0.85) is much higher than that of aa with Rz (r0=0.61). If this model is applied to each solar cycle, the correlation coefficient will be higher (rf=0.95). This model can naturally explain some phenomena related to aa and Rz, such as (i) the significant increase in the aa index (and its baseline) over the twentieth century; (ii) the longer lag times of aa to Rz at solar cycle maxima than at minima; and (iii) the variations in the correlations related to solar and Hale cycles. These results demonstrate that aa depends not only on the present Rz but also on past values. The profile of aa can be better predicted from Rz by this model than by point-point correspondence.
Highlights
Studying the relationship between solar and geomagnetic activity is useful for understanding the origin and formation of the latter
The geomagnetic activity has been found to be well correlated with the solar wind speed (v), the southward component (Bz) of the interplanetary magnetic field (IMF) and the product Bzv2 (Snyder et al, 1963; Russell and McPherron, 1973; Garrett et al, 1974; Crooker et al, 1977; Svalgaard, 1977; Feynman, 1980; Tsurutani et al, 1988; Wang and Sheeley, 2009)
The data used in this study are the time series of monthly mean aa geomagnetic index, representing the geomagnetic activity (Mayaud, 1972), of the reliable values since 18681 together with the monthly mean sunspot number (Rz)2, representing the solar activity
Summary
Studying the relationship between solar and geomagnetic activity is useful for understanding the origin and formation of the latter. The geomagnetic activity has been found to be well correlated with the solar wind speed (v), the southward component (Bz) of the interplanetary magnetic field (IMF) and the product Bzv (Snyder et al, 1963; Russell and McPherron, 1973; Garrett et al, 1974; Crooker et al, 1977; Svalgaard, 1977; Feynman, 1980; Tsurutani et al, 1988; Wang and Sheeley, 2009). The increasing occurrence of high-speed solar wind streams during the declining phase of the cycle, whose reason needs to be explained, has been used to explain the decreasing trend in the correlation between aa and Rz over time (Bame et al, 1976; Borello-Filisetti et al, 1992; Mussino et al, 1994; Tsurutani et al, 1995; Kishcha et al, 1999).
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